The present invention relates generally to the measurement of fluid viscosity and more particularly to a viscosity tester which orbits a free standing object (e.g., one for which the top portion is not constrained) through fluid placed in a testing cup and, sensing movement of the object around the cup, provides data from which the viscosity may be calculated.
The accurate measurement of the viscosity of a fluid is important in a variety of commercial and scientific endeavors. Accordingly, a number of viscosity testers and testing methods have been developed, with the goal being a viscosity measurement that can be done quickly and which produces accurate and repeatable results for fluids having a wide range of viscosities. Unfortunately, the prior art has not fully met this goal.
One measurement technique that has been tried repeatedly in the prior art is to cause the movement of an object through the fluid to be tested under conditions whereby the speed or velocity of the object can then be determined. If proper computational techniques are used, the viscosity of the fluid can then be calculated from the velocity measurement.
Thus, for example, the viscometer disclosed in U.S. Pat. No. 4,864,849 moves a ferromagnetic bob in a reciprocating linear fashion through a tube which, in fact, is intended to be part of and not easily removable from an oil lubrication system. Thus, the device of the '849 patent is not adapted for efficient measurement of fluid viscosity in a shop or in applications where a number of different fluid samples must be checked.
The invention of Russian Invention Certificate No. 1,272,181 measures the viscosity of plastic media, particularly concrete mixtures, by placing in rotary motion a vessel containing a ball, within the mixture to be measured. A rotating magnetic field moves the ball through the mixture in a spiral trajectory having a diameter limited by the interior surface of the cylindrical vessel. The amount of time required for the ball to travel a predetermined path is then measured and from which viscosity is calculated. An apparent shortcoming of the Russian '181 viscometer is that it relies on actual mechanical movement of a rotating vessel in conjunction with a non-linear movement of the ball to produce the data from which viscosity is calculated. It requires two sensors, measures only the time of a partial revolution, and does not repetitively detect speed of the ball. The two sensors are placed in the plane and pitch of the spiral trajectory of the falling ball, and the speed of the ball is measured along an arc as it falls by the sensors. Also, there is no teaching of a testing method or apparatus which would be applicable to fluids having a wide range of viscosities, including non-plastic media.
The invention of U.S. Pat. No. 4,643,021 measures the viscosity of blood by constraining a cylinder at both ends, immersing the cylinder within a close fitting chamber surrounded by fluid, applying known torques to the cylinder through a magnetic field, and measuring the resulting speed of the cylinder at each torque level. The test chamber is confined at the top, bottom, and outside diameter. This precludes rapid removal and cleaning of the cylinder or chamber except by flushing with solvent. This invention is uniquely intended for low viscosity biological fluid such as blood.
What is needed, then, is a low cost, rapid, simple viscosity tester and method which can easily and quickly be used in the shop or field or laboratory or in-line to measure the viscosity of fluids having a wide range of viscosities and which does so with good repeatability, reasonable accuracy, small sample volume, and solvent free cleaning. Such a device is lacking in the prior art.